It depends, I'm on a level 3 BTEC and if I left now in my second year then I'd come out with nothing because I never completed the course. If you've moved on to a higher level this year then it's a different course and that grade will be final.
You can resit the exam in summer if it goes really bad plus BTEC is quite flexible, I messed up my first year with nothing above a pass but all I need is 2D and 4M to get MM, highest I can get is DM. Take it from a 22yo in college because I dropped out in the last 6 months instead of pushing through to see what I could get, don't quit. I've had to take out a £8,000 student loan to be back in college, it's worth trying even if you don't get perfect grades - bad ones are better than none.